Aromatic polyamide fibers, known as aramid fibers, are synthetic fibers obtained by polycondensation and spinning of aromatic compounds as raw materials. Aramid fibers typically include polyterephthaloyl p-phenylene diamine fibers and polyisophthaloyl metaphenylene diamine fibers. The polyisophthaloyl metaphenylene diamine fibers exhibit excellent and persistent thermal stability, remarkable flame retardance and outstanding electrical insulation properties, since they have a conjugated structure formed by a meta-benzene ring and an amide bond on the main chain of the molecule, and have hydrogen bond interaction between the molecules. Therefore, the polyisophthaloyl metaphenylene diamine fibers find a wide range of applications in specialized protective clothing, high temperature filtration materials and honeycomb flame retardant materials.
Presently, one approach for preparing the polyisophthaloyl metaphenylene diamine fibers is an interfacial polymerization process which results in a powdered polymer, the powered polymer is dissolved with a cosolvent in an amide solvent, and then spun. For example, this approach is reported in U.S. Pat. No. 3,640,970. However, this approach is operating intermittently and hard to obtain a stable solution.
Another approach for preparing the polyisophthaloyl metaphenylene diamine fibers is a low temperature solution polymerization process, in which polyisophthaloyl metaphenylene diamine is prepared by a polymerization reaction in an amide solvent at a low temperature, with a side product of hydrogen chloride which is neutralized in the solution to produce salts, the polymer solution containing the salts are directly used for dry-spinning. This approach can be found in U.S. Pat. No. 3,063,966. Due to the production of high contents of salts, wet spinning would lead to fibers with low mechanical properties, and therefore is not applicable to this approach.
Moreover, attempts have been made to utilize different types of neutralizing agents to neutralize the polymerization side product, hydrogen chloride, for example in Chinese patent application No. CN1341169A. This application teaches a neutralizing agent that can be reacted with hydrogen chloride to give a solvent-insoluble salt which can be removed by filtration. With this process, a polymer solution with low content of salts can be obtained. However, the polymer has a relatively high viscosity, thus is difficult for the filtration operation. In addition, the filtration would be accompanied with loss of plenty of the polymer solution, and the production procedures of this process are complicated.
Chinese patent application No. CN101285214A discloses a process for preparing aramid 1313 fibers by wet spinning, comprising pre-polymerizing m-phenylenediamine (MPD) with isophthaloyl chloride (IPC) to give a prepolymer which is filtered and extruded through a twin screw extruder; completely neutralizing hydrogen chloride produced by the pre-polymerization using calcium hydroxide to obtain a spin dope; and wet spinning the spin dope to produce the finished fibers.
Another Chinese patent No. CN100455706C discloses a process for producing a spin dope of polyisophthaloyl metaphenylene diamine, comprising polymerizing m-phenylenediamine with isophthaloyl chloride in a polar organic amide solvent at a low temperature, and neutralizing hydrogen chloride produced by the polymerization using alkaline earth metal compounds to give alkaline earth metal chlorides which are soluble in the reaction system.
The current technologies including the approaches and processes discussed above have a common feature that the side product as a result of the neutralization reaction, hydrogen chloride, is required to be removed completely through a complete neutralization reaction. This can avoid the degradation of the spin dope containing hydrochloric acid before it is fed to the spinning station, but a great number of salts produced by the neutralization reaction would be included in the spin dope. It is well known in the art that the inclusion of the salts may make a strong impact on the properties of the finished fibers, for instance with a result of poor electrical insulation and increased grayscale of the fibers. As a consequence, washing thoroughly the fibers is needed in order to remove the salts. However, repeated washing of the fibers not only consumes a vast amount of water and wastes valuable resources, but also prolongs the production process, and more importantly, removing the salts by washing the fibers is not very effective.
The invention is developed to tackle the above technical problem by providing a method for preparing meta-aramid fibers which is effective to shorten the production time and reduce significantly the production costs. According to the method of the invention, the spin dope contains a little amount of salts which can be completely removed easily by the washing operations. Thus, both the costs of raw materials and the energy consumption necessary for preparing the fibers are reduced.